Title:
Understanding the "anti-kick" in the merger of binary black holes

Abstract: The generation of a large recoil velocity from the inspiral and merger of
binary black holes represents one of the most exciting results of
numerical-relativity calculations. While many aspects of this process have been
investigated and explained, the "antikick", namely the sudden deceleration
after the merger, has not yet found a simple explanation. We show that the
antikick can be understood in terms of the radiation from a deformed black hole
where the anisotropic curvature distribution on the horizon correlates with the
direction and intensity of the recoil. Our analysis is focussed on
Robinson-Trautman spacetimes and allows us to measure both the energies and
momenta radiated in a gauge-invariant manner. At the same time, this simpler
setup provides the qualitative and quantitative features of merging black
holes, opening the way to a deeper understanding of the nonlinear dynamics of
black-hole spacetimes.